Conventionally, various image processing apparatus for processing images in a digital manner have been developed. In the image processing apparatus, it is simple to binarize two-tone images, such as coded data images, graphic images and character images. That is, the two-tone images can be simply binarized in reference to a prescribed threshold value. Generally, the system is called a simple binarization system. However, it is rather difficult to binarize continuous-tone images, such as photograph images. Thus, many techniques or systems for binarizing the continuous-tone images, e.g., a quasi gradation system such as a dither system, have been proposed. The dither system operates to minimize a distortion of image data.
However, both the binarizations of the two-tone images and the continuous-tone images (the images will be referred to as character images and photograph images, hereafter) are mutually inconsistent. That is, a gradation of the photograph images is deteriorated when the simple binarization system is employed. On the other hand, a resolution of the character images is deteriorated when the quasi gradation system is employed. Thus, in the practical apparatus the two systems are selectively employed by a manual selection. However, either the character images or the photograph images are inevitably damaged when the character images are and the photograph images mixedly present on the same document.
To solve the problem, an Error Diffusion System has been proposed in the article "An Adaptive Algorithm for Spatial Grey Scale" in the magazine titled "Proceeding of the S.I.D.", Vol. 17/2 Second Quarter 1976, pp. 75-77. This system has made the requirements of the gradation of the photograph image and the resolution of the character image be compatible. That is, according to the Error Diffusion System binarization, errors occurring in a binarizing process of every pixel which is actually processed (the actually processed pixel will be referred to as subject pixel hereafter) are diffused on other pixels around the subject pixel after the binarization errors were each multiplied with a predetermined weighting factor.
However, a problem still exists with the Error Diffusion System when the continuous-tone images and the two-tone images are mixedly present on a same document because a resolution of the two-tone images is deteriorated.